Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the current. Displays using inorganic light emitting diodes (LEDs) are also in widespread use for outdoor signage and have been demonstrated in a 55-inch television.
Inorganic light-emitting diode displays using inorganic micro-LEDs on a display substrate are also known. Micro-LEDs can have an area less than 1 mm square, less than 100 microns square, or less than 50 microns square or have an area small enough that it is not visible to an unaided observer of the display at a designed viewing distance. U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate.
In any application requiring many elements, it is important that each element is reliable to ensure good manufacturing yields and performance. Active-matrix control circuits, as well as the controlled element (e.g., a light emitter) are subject to failure. Because no manufacturing process is perfect, any large system can have defective elements. In particular, inorganic light-emitting diodes are subject to manufacturing defects that increase the amount of current passing through the LED when provided with power, resulting in an undesirable brightness, power usage, or system control problems.
An undesired increase in current can also overheat the LEDs. One approach to preventing such overheating is to provide each LED pixel with a resistor or group of resistors in series with the LED, as is described in U.S. Pat. No. 9,226,361. U.S. Patent Application Publication No. 20160057827 discloses a current-limiting diode in an LED circuit for a lighting system with both forward-biased and reverse-biased LEDs and a polarity switching device. Circuits for sensing current levels and reducing power dissipation are also known, for example as taught in U.S. Patent Application Publication Nos. 20160057822 and 20160057832 as are voltage control circuits, for example as described in U.S. Patent Application Publication No. 20160056725. However, these approaches either require complex or expensive circuit components in association with each LED or result in diminished light output.
Alternatively, to ensure that large multi-element systems are reliably manufactured and operated, such systems can employ redundant elements. For example, displays are sometimes designed with redundant light emitters. U.S. Pat. No. 5,621,555 describes an LCD with redundant pixel electrodes and thin-film transistors to reduce defects. In another approach described in U.S. Pat. No. 6,577,367, an extra row or column of pixels is provided to replace any defective row or column. An alternative approach to improving display yields uses additional, redundant light-emitting elements, for example two light emitters for every desired light emitter in the display. U.S. Pat. No. 8,766,970 discloses a pixel circuit with two sub-pixels and circuitry to determine whether a sub-pixel is to be enabled, for example if another sub-pixel is faulty. Similarly, U.S. Pat. No. 7,012,382 teaches an LED-based lighting system that includes a primary light source and at least one redundant light source. The primary light source is activated by itself and the performance of the light source is measured to determine whether or not to drive the redundant light source. The redundant light source is activated when the performance measurements indicate that a performance characteristic is not being met by the primary light source alone. The first light system can be activated in combination with the redundant light source once the decision is made to activate the redundant light source. U.S. Pat. No. 8,791,474 discloses redundant pairs of micro LED devices driven by a common transistor. WO 2014149864 describes separately controlled micro-LED devices. However, the use of redundant emitters is expensive and does not address problems with LEDs that conduct too much current.
There is a need, therefore, for LED pixel circuits that can control or avoid problems resulting from LEDs in a display that undesirably conduct too much current.